1. Field of the Invention
The present invention relates to methods and apparatus for surface and wavefront metrology; more particularly to methods for measurement of surface locations with respect to machine motion axes and the measuring device; and most particularly, to a method for calibration of the geometry of an integrated metrology system comprising a multi-axis CNC mechanical positioning apparatus and an embedded wavefront-measuring gauge.
2. Discussion of the Related Art
In applications of single-point scanning measurement systems and machining, it is extremely important to achieve accurate positioning. Positioning errors in a sensitive direction translate into one-to-one cutting/measuring errors. As a result, considerable effort has been devoted to the manufacture of accurate positioning systems, methods for measuring such accuracies, as well as means for compensation of motion biases such as axis non-orthogonality. Such methods can include laser position measuring equipment on an external metrology frame. Micrometer level and better positioning accuracies can be achieved with such methods.
Such methods can be quite expensive, both in terms of machine design and cost of metrology equipment. They also often require a high degree of environmental control for best results. Furthermore, such methods are typically applied only to translational (X-Y-Z) motion. Spindles, when present in such a machine, are typically used for rotating a cutting tool at high speed, or rotating the part with respect to a cutting tool. Scanning measurement tools, such as profilometers and coordinate-measuring machines (CMMs), rarely have rotational axes. Those few that do are typically for azimuthal scanning of a nearly rotationally symmetric surface.
Wavefront-measuring gauges (such as a Fizeau interferometer or Hartmann-Shack wavefront sensor), however, operate in a different paradigm from that of machine tools and scanning measurement apparatus. Such gauges acquire a plurality of measurement points on a surface—often the entire test surface—as opposed to a single localized point that is scanned along the test surface. As a result, the part positioning and stage alignment requirements needed in wavefront measuring gauges is comparatively crude.
Wavefront-measuring gauges commonly have much less longitudinal measurement range than a profiling instrument. As a result, the part must be positioned more precisely than for a profiler, and often require tilting motions that profilers do not. Accuracy of the motions, however, is not important as the wavefront measuring device provides its own feedback as to the part's relative position to the gauge. Also, data acquisition time for a wavefront-measuring gauge is short compared to a profiler: seconds, or even faster, as compared to minutes and longer. The long-term absolute stability of the part and measurement gauge is thus less important for wavefront-measuring gauges. Another key difference is that profilers require motion during the measurement acquisition.
To summarize, profilers and high-precision CNC machining centers typically require dynamic motion with high accuracy on three translational axes. Wavefront-measuring gauges typically require tilt positioning for the part, but lack the high accuracy and dynamic requirements.
Accurate positioning in a metrology system comprising a wavefront-measuring gauge and mechanical positioning apparatus, though not necessary for general use, does have several applications. Some examples include calibration methods that employ multiple part motions, measurement of a part or system at an angle with respect to the wavefront-measuring gauge, and subaperture stitching of multiple measurements taken at different positions on a test surface.
Such applications require more accurate motion than a “usual” system. It is known in the prior art to apply methods used for profilers and high precision machining to align and calibrate translational axes of motion by use of indicator gauges, laser displacement gauges, and other such means. Such calibration, however, can be quite expensive and time-consuming, and is not readily applicable to the rotary axes necessary for a wavefront-measuring metrology system. Furthermore, such methods reveal nothing about centration of the embedded gauge with respect to the mechanical axes and stage, particularly the rotary axes. It is known to measure part wedge/taper and centration, or to eliminate it with respect to a spindle axes via manual alignment or shimming, but this requires additional equipment and/or tedious steps.
What is needed is a method for calibration of the geometry of an integrated metrology system using the wave-front measuring gauge embedded in the complete system.
What is further needed is a method for locating a specified part surface coordinate to a specified embedded gauge coordinate.
What is further needed is a method for measuring wedge and/or decentration of a mounted part with respect to a spindle axis.
It is a primary objective of the present invention to measure the geometric relationships in a complete metrology system between a wave-front measuring gauge, a test part surface, and mechanical axes to the accuracy necessary for a wavefront measurement.
It is a further objective of the present invention to calibrate and align the metrology system in preparation for moving a test part surface such that a specified part surface coordinate is located to a specified embedded gauge coordinate without any additional expensive metrology equipment, inbuilt metrology systems, or time-consuming alignments.
It is a still further objective of the present invention to measure the wedge and/or decentration of a part with respect to a spindle axis, further enabling precision centration of the part.